Voltage controlled oscillators (VCO's) of this type can be used as independent voltage to frequency converters, but are especially used as parts of control systems such as phase-locked loops (PLL's).
A VCO of the type mentioned hereinbefore is known from U.S. Pat. No. 4,146,849. In this known VCO the feedback loop comprises a third signal inverter for coupling the junction of the capacitor and the resistor, which are connected to the output of the first and the second signal inverter, respectively, to the input of the first signal inverter. The three signal inverters are designed in complementary MOS technology (CMOS), the second CMOS signal inverter being arranged between complementary MOS field effect transistors (CMOS FET's) which are connected to the respective supply voltages and function as resistors whose values can be modified with the aid of two oppositely varying control voltages at their gate electrodes. Thus, the output resistance of the second CMOS signal inverter can be modified externally and hence the frequency of the signal generated by the VCO. At a given value of the control voltages, however, the oscillation frequency still depends rather strongly on the properties of the CMOS FET's and, more specifically, on the so-called "pinch-off voltage", the threshold of the gate voltage at which a FET just begins to be conductive. For a FET having the same channel type as well as for the FET's having a complementary channel type, this "pinch-off voltage" shows a relatively large variation, whilst the deviations with respect to the nominal value can be .+-.0.8 Volt in practice and thus the accidental value of the difference may rise to 1.6 Volts. In addition, with this VCO the charging and discharging of the capacitor in each oscillation period takes place via the output resistor of the second CMOS signal inverter which is determined during the charging operation by the resistance of the FET of the one channel type and during the discharging operation by the resistance of the FET of the other channel type. For obtaining the practically desired symmetrical signal shape of the oscillator signal (that is to say, a signal having a duty factor of 50%) additional provisions are required for this VCO in order to neutralize accidental inequalities in the charging and discharging resistors.